The present invention relates generally to a change-over valve and, more particularly, to a change-over valve for producing time overlapped operation of a plurality of fluid pressure actuators.
In many applications a member is reciprocated or stopped at suitable positions by a reciprocating type hydraulic actuator. Typically, a piston is inserted into a cylinder to define a pair of oil chambers and a rod coupled to the piston extends externally from the cylinder. A general direction change-over valve of that type is shown in FIG. 2. More specifically, when a spool 18 is moved rightwardly (position shown in FIG. 2) from a neutral position, pressure oil is supplied from a hydraulic pump, not shown, to an input port 19 via a passage 7. In addition, pressure oil is supplied to a chamber at one end of an actuator via a central annular groove 20 of a spool 18, an output port 23 and a passage 8 producing forward movement of the hydraulic actuator. At the same time, oil in a chamber at the other end of the hydraulic actuator is introduced into an output port 17 via a passage 4 and then returned to an oil tank, not shown, via an annular groove or small diameter portion 26 on opposite ends of the spool 18 and a drain port 16. When the spool 18 is returned to the neutral position, a portion between the input port 19 and each of the output ports 17 and 23 is cut off by a pair of lands on the spool 18, and the hydraulic actuator is stopped at that position. Conversely, when the spool 18 is moved leftwardly from the neutral position, the direction of oil flow is reversed so that the hydraulic actuator is moved backward.
In the case where a single hydraulic actuator is driven by a change-over valve as described above, the hydraulic actuator is operated in the reverse direction immediately when the spool is switched. However, in the case where a pair of spring biased hydraulic actuators are driven by a change-over valve, operation of members driven by the pair of hydraulic actuators does not occur without an intermittent time delay. For example, a speed change gear or transmission for an agricultural tractor, a construction vehicle and the like is generally provided with a hydraulic type low-speed shift clutch and a hydraulic type high-speed shift clutch. When pressure oil is supplied to one of these hydraulic actuators, one of the clutches is connected. In a speed change gear of this kind, when the change-over valve is switched, for example, in order to disconnect the low-speed shift clutch and connect the high-speed shift clutch, the low-speed shift clutch is immediately disconnected but a delay occurs before pressure oil is fully supplied to the hydraulic actuator for the high-speed shift clutch. During the delay period, the vehicle speed is lowered or stopped followed by a very unsmooth up-shift.
The above described operation is illustrated in FIG. 3 which shows hydraulic actuators 31 and 32 for connecting and disconnecting a high-speed shift clutch 33 and a low-speed shift clutch 33a. A piston 36 is inserted into a cylinder 35 to define an oil chamber in one end thereof while a spring 34 is disposed on the other end to bias clutches 33 and 33a connected to the piston 36 through a rod 40 in a disconnecting direction. In the state illustrated in FIG. 3, pressure oil from a hydraulic pump 38 is supplied to an oil chamber of the hydraulic actuator 32 of the low-speed shift clutch 33a via an electromagnetic change-over valve 37 and oil in an oil chamber of the hydraulic actuator 31 of the high-speed shift clutch 33 is returned to an oil tank 39. When the electromagnetic change-over valve 37 is switched, the oil pressure of the hydraulic actuator 32 is released to the oil tank 39 via the electromagnetic change-over valve 37 and the low-speed shift clutch 33a is immediately disconnected. However, the high-speed shift clutch 33 is not connected until the pressure oil from the hydraulic pump 38 enters the oil chamber of the hydraulic actuator 31 to produce a stroke by the piston 36. Unsmooth operation occurs because if the load on the vehicle is high, the vehicle speed drops substantially after disconnection of the low speed clutch during shift and when the high-speed shift clutch 33 is connected, the vehicle quickly accelerates.
The object of the present invention, therefore, is to provide a change-over valve which can impart smooth operation without an intermittent delay period to members driven by a plurality of fluid pressure actuators.